Method for delayed coking of oil residues

ABSTRACT

The invention relates to the petroleum-processing industry, in particular, to a retarded coking process directed to the production of gas oil fractions (light gas oil and two types of heavy gas oil). The method comprises heating a starting stock, feeding same into an evaporator for mixing with recycled material and forming a secondary stock, coking the secondary stock and fractionating the resulting coking products in a rectification column, thereby producing gas, petrol; light and heavy coking gas oils, and bottoms. Heavy coking gas oil is used as the recycled material. The method envisages feeding cooled heavy gas oil to mass-exchange devices in the lower part of the rectification column, and feeding a metered quantity of cooled light gas oil to mass-exchange devices in the upper part of the rectification column. The quantity of cooled light and heavy gas oils which can be fed in changes depending on the required quantity and quality of the light and heavy gas oils and bottoms being produced.

The invention relates to the oil-refining industry, in particular to theprocess of delayed coking with the purpose of obtaining oil coke and gasoil fractions.

The exploitation of many delayed coking installations, particularly inthe reprocessing of oil residues, from which coke with high sulfurcontent is obtained in the process of coking, is oriented to obtainingthe maximum possible quantity of distillated fractions: gasoline, lightand heavy coking gas oil. Further on, high quality motor fuels areproduced from the obtained distillates by using hydro-catalyticprocesses—hydraulic cleaning, catalytic cracking, and hydrocracking.Thereby the obtained coking products must correspond to certain qualityrequirements. The coking gasoline (fraction initial boiling point 180°C.) is sent for hydraulic cleaning and is then subjected to reformingwith obtaining a high-octane component of commercial gasoline. The lightcoking gas oil (fraction 180-350° C.) is subjected to hydraulic cleaningwith obtaining diesel fuel. That is why there are requirements to thelight gas oil with respect to the end boiling point because the highcontent of high-boiling fractions in it contributes to the carburizationof the hydraulic cleaning catalyst and its premature regeneration. Theheavy coking gas oil (fraction >350° C.) is subjected to hydrauliccleaning in the course of the coking of the sulfur feed stock and thento catalytic cracking with obtaining high-octane gasoline or tohydrocracking with obtaining diesel fuel. That is why there are strictrequirements also to the heavy gas oil with respect to the end boilingpoint because the high-boiling fractions in it, which determine the highlevel of the coking capacity, lead also to premature carburization ofthe used catalysts. There are requirements to the heavy gas oil alsowith respect to the bubble-point temperature because, first, thelow-boiling fractions, which are boiling off at temperature of up to350° C., diminish the yield of the light gas oil which is used for theproduction of diesel fuel; second, these low-boiling fractions are aballast in the hydro-catalytic processes, they overload the installationand reduce the end products yield. There are particularly strictrequirements to the heavy gas oil and the bottom liquid with respect tothe content of coke particles, the so-called carboides, which aredeposited on the surface of the catalysts of the hydro-catalyticprocesses. If the carboides content is high (for example >0.1%), thensuch product is not usable for hydro-catalytic processing and can beused only as a component of boiler fuel.

In order to increase the production of distillates, the installationsfor delayed coking are exploited with the minimum possible (sometimeseven with a zero) recirculation ratio and at the minimum possiblepressure in the coking chamber. The operation of the installations fordelayed coking at such technological parameters significantlycontributes to increasing the yield of the heavy coking gas oil(fractions boiling off at temperatures above 350° C.). However, alongwith that, the end boiling point temperature increases significantly,the coking capability, as well as the content of high molecular weightpolycyclic aromatic hydrocarbons, resins, asphaltenes and metal-organiccompounds also increase. These high molecular hydrocarbons contained inthe tail fractions of the heavy coking gas oil determine the quickdeactivation of the catalysts used for hydraulic cleaning andhydrocracking.

A method for delayed coking of oil residues is known, according to whichit is proposed to increase the amount of the fed “cold” heavy gas oil inthe bottom portion of the column as reflux for the purpose of regulationof the end boiling point temperature of the heavy coking gas oilobtained from the rectification column and as a result of that forimprovement of its quality [G. M. Sieli, A. Faegh, S. Shimoda, “Fineregulation of the coking behavior”.—Oil-Gas Technologies, 2008, No. 1,p. 74-77]. As a result, the highest boiling fractions contained in theheavy gas oil are condensed, form recirculate and along with the fresh(primary) raw material at the bottom of the rectification column are fedthrough the furnace as secondary raw material into the coking chamber.In this way, a portion of the highest boiling fractions is removed fromthe heavy coking gas oil, which leads to improvement of its quality andhas a favorable effect of the subsequent hydro-catalytic processing forobtaining motor fuels.

An essential shortcoming of this method is the circumstance that despitethe fact that when the amount of cold reflux is fed into the bottom part(cube) of the rectification column, the end boiling point temperature ofthe heavy coking gas oil decreases, at the same time the recirculationratio increases, which leads to higher energy consumption, reduces thefresh raw material productivity and decreases the yield of heavy gasoil. In addition to that, the coke particles, which are introduced intothe rectification column from the coking chambers, are dragged into thesecondary raw material and after getting into the furnace they cause itspremature carburization.

The closest method to the object of the application is the method fordelayed coking of oil residues, including heating of the source rawmaterial, its division into light fraction and heavy residue in thevaporizer, fractioning of the light fractions in the rectificationcolumn together with the vapor-liquid products of the coking process,mixing of the heavy residue from the vaporizer with therecirculate—bottom liquid, obtained during the functioning of therectification column with subsequent coking of the preheated mixture.The quantity and quality of the bottom liquid is regulated by changingthe amount of the fed cold heavy gas oil as a reflux for watering themass-exchange devices at the bottom part of the rectification column,whereby the bottom liquid is fed into the upper portion of the vaporizerequipped with mass-exchange devices (Patent of the Russian Federation RFNo. 2209826 from Aug. 10, 2003 MPK C10B55/00).

This method is oriented in the first place towards regulation of thequality (with respect to density, coking capability and fractionalcomposition) of the bottom liquid extracted from the bottom of therectification column and sent to the vaporizer for mixing with thesource raw material as a recirculate. In this method, the bottom liquid,which is formed in the main rectification column, is taken out from thebottom of the column and is fed to the vaporizer as recirculate formixing with the source raw material with the formation of secondary rawmaterial, its heating in the furnace and its feeding into the cokingchambers for coking. The organization of the dosed feeding of thecooling gas oil to the mass-exchange devices in the bottom part of therectification column, the feeding of the bottom liquid from the bottomof the rectification column into the upper portion of the vaporizer, andthe organization of the dosed feeding of steam into the bottom portionof the vaporizer lead to increasing the weight of the residue from thebottom of the vaporizer (the secondary raw material) and as aconsequence to increasing the production of coke and reducing the powerconsumption.

This method has a shortcoming in the sense that in the case of foamtransfer from the working coking chambers the bottom liquid from thebottom of the rectification column may contain coke particles which willbe deposited on the mass-exchange devices in the vaporizer and infringeupon their operation, whereby the coke particles get in the furnacetogether with the secondary raw material, which leads to theircarburization and, therefore, to a shorter time between the overhauls ofthe delayed coking installation. This method does not provide regulationof the quantity and quality of the light gas oil extracted from theinstallation. The change in the amount of the cooling gas oil in thismethod leads to a change in the recirculation ratio.

The objective of the proposed method is to increase the time between theoverhauls of the installation for delayed coking of oil residues bydecreasing the carburization of the mass-exchange devices of thevaporizer and the coils of the reaction furnace by excluding the entryof coke particles in them as well as to obtain two kinds of heavy gasoil with a possibility for regulation of the quality of the obtained gasoil fractions (light gas oil and two kinds of heavy gas oil)independently of the recirculation ratio.

In the method for delayed coking of oil residues, according to theinvention, including heating the source raw material for coking, itsfeeding into the vaporizer for its mixing with the recirculate andforming a secondary raw material, heating the secondary raw material andits feeding into the coking chamber, feeding the products of the cokingfrom the coking chamber into the rectification column for fractioningwith obtaining gas, gasoline, light and heavy coking gas oil as well asbottom liquid, feeding the cooled heavy gas oil to the mass-exchangedevices in the lower part of the rectification column with this achievedby feeding cooled light gas oil to the gas-exchange devices in thebottom portion of the rectification column.

The quantity of the fed cooled light and heavy gas oil is changeddepending on the required quantity and quality of the obtained light andheavy gas oil and bottom liquid.

Heavy coking gas oil is used as recirculate.

In particular, cooled heavy coking gas oil is used as reflux forregulation of the end boiling point of the heavy gas oil, and cooledlight coking gas oil is used for regulation of the bubble-pointtemperature of the heavy gas oil.

The principle diagram of the installation for delayed coking forimplementation of the proposed method is shown in the drawing.

The installation comprises a tube-type furnace 1 for heating the sourceraw material, vaporizer 2 with mass-exchange devices, rectificationcolumn 3, tube-type furnace 4, coking chamber 5, strippings 6, coolers7.

The method is realized as follows. The source raw material is heated inthe tube-type furnace 1 and/or in the heat exchangers and is fed intothe vaporizer 2, the upper part of which receives as recirculate heavycoking gas oil from the main rectification column 3. The obtainedsecondary raw material—a mixture of the source raw material with therecirculate is heated in the tube-type furnace 4 and is fed into one ofthe alternately operating coking chambers 5. The distillate productsformed as result of the coking are fed through the overhead gas lineinto the rectification column 3 for fractioning. Gas and unstablegasoline are extracted from the top of the column. In order to regulatethe quality of the gasoline, a portion of the unstable gasoline is sentto the first tray in the form of live reflux. Light and heavy coking gasoil is extracted in the form of off-stream from the main rectificationcolumns through the strippings 6. The major part of the heavy gas oilfrom the stripping 6 and after the cooler 7 is extracted from theinstallation in the form of a finished product, and a portion of it isfed into the overhead gas line as coolant. Cooled-off heavy gas oil fromthe accumulator of the column 3 is fed into the first mass-exchangedevice in the bottom part of the rectification column as flush liquidand for regulation of the end boiling point of the heavy coking gas oil.A regulated amount of the cooled-off light coking gas oil is fed to themass-exchange devices of the upper part of the rectification column 3for regulation of the bubble-point temperature of the heavy gas oil and,consequently, the end boiling point of the light coking gas oil.

The quantity of the fed cooled light and heavy gas oil as reflux ischanged depending on the required quantity and quality of the obtainedlight and heavy gas oil and bottom liquid.

After the cooling, the bottom liquid is extracted from the bottom of therectification column 3 as a commercial product.

In this way, the proposed method permits to obtain and to regulate thequality of two heavy gas oil fractions at the same time: the heavycoking gas oil, which can be sent for hydrocracking or hydrauliccleaning and further for catalytic cracking, and the bottom liquidwhich, depending on its quality, can be used either as raw material forthe hydro-catalytic processes or as a component of boiler fuel. Therebythe recirculation ratio does not change.

The increase in the reflux amount in the bottom part of therectification column promotes the condensation of the heaviest boilingfractions from the coking distillate coming from the coking chambers. Asa result of that, the end boiling point temperature of the heavy gas oilfalls down and its density, coking capability and the content ofpolycyclic aromatic hydrocarbons decrease. Along with that, as a resultof the dilution of the heavy gas oil, a relief takes place in thefunctional composition of the bottom liquid extracted from the bottom ofthe column, whose density, coking capability and the carboides contentare also decreased.

The organization of the feed of the regulated amount of the cooled lightgas oil to the mass-exchange devices of the upper part of therectification column permits to regulate the bubble-point temperature ofthe heavy coking gas oil and, respectively, the end boiling point of thelight gas oil.

Thereby the regulation of the qualitative characteristics of the fedcoking gas oils is not connected with a change of the recirculationration because the quantity of the recirculating fractions drawn intothe coking is strictly regulated by the amount of the heavy coking gasoil fed into the vaporizer.

The method is illustrated by the following examples.

Example 1 (Based on a Prototype)

In an industrial installation, raw material was coked, whosecharacteristics are presented in table 1. The raw material is heated inheat exchangers up to a temperature of 270° C. and is then fed into thebottom portion of the vaporizer. Here, in the upper part of thevaporizer, bottom liquid as recirculate is fed from the mainrectification column in a quantity of 10% with respect to the rawmaterial. The secondary raw material formed in such a way in thevaporizer is heated in the furnace to a temperature of 500° C. and isfed into one of the coking chamber for coking. The products of thecoking in the coking chambers are fed through the overhead gas line intothe lower part of the main rectification column. For the condensation ofthe heaviest boiling products of the coking in the top mass-exchangedevice in the lower part of the rectification column, the feeding ofcold heavy coking gas oil is organized as lower reflux in the amount of10% with respect to the source raw material. All condensed heavy boilingproducts of the coking (bottom liquid) are sent to the vaporizer asrecirculate for mixing with the source raw material, and thenon-condensed light boiling products are fed into the upper part of therectification column for fractioning with obtaining of gas, gasoline,light and heavy coking gas oils. The time between the overhauls was 275days. The yield and the quality of the gas oil fractions obtained in theprocess of coking as well as the length of time between the overhaulsare provided in table 2.

As result of the coking, the yield of the heavy gas oil was 28.2%; ithas low density and low level of coking capability, but it contains 37vol. % fractions boiling off up to 350° C., which had a negative effecton the yield of the light gas oil (27.1 mass %) used as raw material forthe production of diesel fuel.

Example 2 (Based on the Proposed Method)

According to the technological diagram shown in the drawing, coking ofoil raw material, whose characteristics are shown in table 1, wasperformed. The given raw material is heated in heat exchanges to atemperature of 270° C. and then it is fed into the vaporizer, on whosetop tray heavy coking gas oil as recirculate is fed in amount of 10%with respect to the source raw material. The obtained secondary rawmaterial from the bottom of the vaporizer is heated in the furnace to atemperature of 500° C. and is then fed into one of the coking chambersfor coking. The products of the coking are fed into the bottom part ofthe rectification column through the overhead gas line. For regulationof the end boiling point temperature of the heavy coking gas oilextracted from the installation, feeding of cooled heavy gas oil aslower reflux in the amount of 10% with respect to the source rawmaterial is organized to the first mass-exchange device of the bottompart of the rectification column. For regulation of the bubble-pointtemperature of the heavy coking gas oil in the mass-exchange devices inthe upper part of the rectification column, feeding of cooled lightcoking gas oil in the amount of 20% with respect to the source rawmaterial (top reflux) was organized. The yield and the quality of thegas oil fractions obtained thereby as well as the length of time betweenthe overhauls are provided in table 2.

As it can be seen from the table, the yield of heavy gas oil compared tothe prototype (example 1) has decreased a little bit, but theorganization of the feeding of top reflux has led to reducing thecontent of the fractions in it, which are boiling off to a temperatureof up to 350° C., from 37 vol. % to 20 vol. %. Such heavy gas oil can beused as a raw material for obtaining motor fuels by means ofhydrocracking.

In addition to that, the yield of the light gas oil has increasedcompared to the prototype.

From the bottom part of the rectification column, bottom liquid in theamount of 7% with respect to the source raw material is extracted,which, taking into account the insignificant content of carboideparticles in it, can be used for obtaining diesel fuel by means ofhydrocracking.

In this way, the given example shows that the proposed method permits toincrease not only the yield of light gas oil, but also the total yieldof heavy gas oils (heavy coking gas oil and bottom liquid) up to 30.4%(in the prototype, the heavy gas oil yield is 28.2%) with guaranteedquality characteristics.

Example 3 (Based on the Proposed Method)

In this case, the task was to increase the yield of heavy gas oilcompared to example 2, which, due to its quality, could be used as rawmaterial for the hydrocracking.

To this purpose, coking of the source raw material, analogical toexample 2, was performed, but the amount of heavy gas oil, which was fedfor refluxing the mass-exchange device in the bottom part of therectification column of up to 5 vol. %, was reduced, whereby the lightgas oil amount, which was fed for refluxing the mass-exchange device inthe upper part of the rectification column has remained the same as inexample 2.

The results of the coking are presented in table 2.

As one can see from example 3, the reduction of the lower refluxquantity compared to example 2, i.e. the reflux fed to the firstmass-exchange device in the bottom part of the rectification columnleads to increasing the weight of the fractional composition of then fedheavy coking gas oil, which is due to the lower condensation of thehigh-boiling fractions from the coking distillate: If in the case offeeding 10% of lower reflux the content of the fractions boiling off ata temperature higher than 500° C. was 10 vol. %, then when 5% lowerreflux is fed, the content of the fractions boiling off at a temperaturehigher than 500° C., has increased up to 14%. However, at the same time,when the amount of the fed top reflux is unchanged, the content in theheavy gas oil, according to example 3, of the fractions boiling off at atemperature up to 350° C. was reduced by 20 vol. % (example 3) up to 16vol. %, which increases the yield of heavy coking gas oil. Thereby, atthe same time, the weight of the bottom liquid has increased and itsyield has decreased. The fairly high carboide content in the bottomliquid (0.12%) does not permit to use it in a pure form as raw materialfor the hydro-catalytic processes due to the possible deactivation ofthe catalysts. That is why it can be used either as a component of theboiler fuel or as raw material for the cracking after its preliminarymixing with the heavy gas oil, extracted from the side portion of thecolumn, for decreasing the concentration of the carboides.

Example 4 (Based on the Proposed Method)

The task was to increase the yield of the light gas oil compared toexample 2 by preserving the quality of the heavy gas oil for its use asraw material for the hydrocracking. The same raw material as in theexamples 1-3 was subjected to coking in a similar manner as in example2. The amount of the heavy gas oil added as reflux to the mass-exchangedevice in the bottom part of the rectification column has remained thesame as in example 2, but at that the quantity of the fed top reflux (ofthe light gas oil) was 15%.

The results of the coking are presented in table 2.

As it can be seen, by decreasing the quantity of the top reflux weincreased the yield of the light gas oil by 1.6% with respect to thesource raw material, increasing its weight by density and end boilingpoint. Thereby, at the same time, the yield of the heavy gas oil hasdecreased and, naturally, its quality has changed: the density hasincreased by 0.9517 g/cm³ up to 0.9541 g/cm³, the coking capability hasincreased from 0.31 to 0.49, the content of the fractions boiling off atup to 350° C. has decreased from 20 vol. % to 9 vol. %.

In this way, it follows from the data in table 2 that the organizationof the two refluxes: with heavy gas oil in the bottom part of therectification column and with light gas oil in the upper part of thecolumn permits to regulate, depending on the production needs, both theyield and the quality of the products extracted from the column.Therefore, when it is required to increase the generation of heavycoking gas oil, for example as raw material for the installation forcatalytic cracking, and at the same time for reducing the generation ofbottom liquid used as a component of the furnace oil, it is necessary todecrease the quantity of the bottom reflux of the heavy gas oil(examples 2 and 3). Thereby, at the same time, the quality of the heavygas oil and the bottom liquid extracted from the rectification columnchange: in the heavy gas oil within the limits of the requirements thatare admissible for the raw material of the catalytic cracking, thedensity and the coking capability increase, the fraction compositionbecomes heavier (the content of the fractions boiling off attemperatures above 500° C.), the density and the coking capability ofthe bottom liquid also increase and its fraction composition becomesheavier.

On the other hand, when it is required, for example, to increase theyield of light coking gas oil used for the production of diesel fuel andat the same time to reduce in the heavy coking gas oil the content ofthe fractions boiling off at temperatures of up to 350° C. (i.e. namelythe diesel fuel fraction), it is necessary to decrease the amount of thetop reflux (examples 2 and 4). Thereby the yield of light coking gas oilwill increase from 29.9% to 31.4% with some weight increase on density,fractional composition, and at the same time the content of thefractions boiling off at temperatures up to 350° C. in the heavy gas oilwas reduced from 20 vol. % to 9 vol. %.

In this way, the use of the proposed method will permit, compared to theprototype, to increase the time between the overhauls of theinstallation for implementation of the delayed coking of oil residuesdue to the fact that the use of the heavy coking gas oil as recirculateand the extraction of the bottom liquid from the bottom part of therectification column, containing coke particles, as a commercial productexcludes the involvement of the bottom liquid in the coking process,prevents the probability for penetrating of coke particles (carboides)into the mass-exchange trays of the vaporizer and in the reactionfurnace and their carburization.

In addition, the proposed method for delayed coking ensures theobtaining of two kinds of heavy coking gas oils which differ in theirphysico-chemical quality characteristics and have concrete areas of use:of the heavy coking gas oil extracted from the side part of therectification column, from which one can obtain motor fuels by means ofhydrocracking or hydrofinishing, with subsequent catalytic cracking, andbottom liquid which, depending on its quality, can be used either as rawmaterial in the hydrocatalytic processes or as a component of the boilerfuel.

In addition, the total yield of the heavy coking gas oils in theproposed method is higher than in the method according to the prototype.This is due to the fact that in the method according to the prototypethe bottom liquid is involved in the coking in a mixture with the sourceraw material with the formation of additional amount of coke, gas andgasoline, while in the proposed method the bottom liquid is extractedfrom the installation in the form of a finished product.

In addition, the proposed method will permit to regulate both thequality and the quantity of the obtained light gas oil, heavy gas oiland bottom liquid, whereby the said regulation is not connected with anychange of the recirculation ratio, whereby, in the method according tothe prototype, when the amount of the cooling gas oil fed to themass-exchange devices in the bottom part of the rectification columnchanges, the recirculation ratio also changes.

TABLE 1 Characteristics of the source raw material for the coking (oiltar mixture of West-Siberian and Arlansk oils) No. Parameter nameParameter value 1 Density, p4²⁰ 1.0360 2 Sulfur content, mass %: 3.9 3Coking capacity, mass % 18.2 4 Funnel viscosity, ° E at 80° C. 952.4 at100° C. 202 5 Fractional composition: up to 460° C. boils off, mass %1.0 up to 500° C. boils off, mass % 4.72 above 500° C. boils off, mass %95.28

TABLE 2 Yield and characteristics of the gas oil fractions extractedfrom the rectification column Example based on a prototype Proposedmethod examples 1 2 3 4 Light Heavy Light Heavy Light Heavy Light Heavygas oil gas oil gas oil gas oil gas oil gas oil gas oil gas oil of of ofof Bottom of of Bottom of of Bottom Parameter name coking coking cokingcoking liquid coking coking liquid coking coking liquid 1. Lower reflux— 20 20 15 duty, raw material % 2. Lower reflux 10 10 5 10 duty, rawmaterial % 3. Product yield of 27.1 28.2 29.8 23.4 7.0 29.8 27.4 3.031.4 21.8 7.0 the fresh raw material from the coking, mass % 4. Density,g/cm³ 0.8509 0.9414 0.8636 0.9517 0.9731 0.8670 0.9583 0.9809 0.86520.9541 0.9731 5. Coking capacity, — 0.15 — 0.31 0.81 — 0.53 1.03 — 0.490.86 mass % 6. Fractional composition: Start of boiling 162 208 172 258350 179 270 390 176 268 360 temperature. ° C. 10% vol. boils off 215 316226 327 386 234 336 400 231 351 391 at temperature. ° C. 90% vol. boilsoff 309 420 343 481 505 350 495 — 347 487 507 at temperature. ° C. Endof boiling 340 512 363 520 — 371 — — 368 520 — temperature. ° C. Up to350° C. boils — 37 93 20 — 90 16 — 91 9 — off. % vol. Above 500° C. — 8— 10 18 — 14 20 — 11 18 boils off. % vol. 7. Carboides — — — — 0.05 — —0.12 — — 0.05 content, mass % 8. Run life of the 275 350 350 350installation, days

1-3. (canceled)
 4. A method for delayed coking of oil residues,comprising the steps of: heating a source of raw material for coking andfeeding it into a vaporizer so as to mix with a recirculate in order toform a secondary raw material for coking; heating the secondary rawmaterial and feeding it into a coking chamber; feeding any products ofthe coking from the coking chamber into a rectification column forfractioning so as to obtain gas, gasoline, light and heavy coking gasoil, and bottom liquid; feeding the heavy coking gas oil that has cooledto a mass-exchange device in a lower part of the rectification column;and feeding the light coking gas oil that has cooled to a gas-exchangedevice in an upper portion of the rectification column
 5. The methodaccording to claim 4, wherein a quantity of the fed cooled light andheavy coking gas oil changes depending on a required quantity andquality of the obtained light and heavy coking gas oils and bottomliquid.
 6. The method according to claim 4, wherein the heavy coking gasoil is used as a recirculate.